The datalink problem, which is solved by the instant invention, has been defined in the past as one having an extremely high data rate and a high incidence of man-made noise in the environment in which it is employed. The problem has been attacked by one of at least two approaches: (1) A frequency hopping (FH) system in which only one of many channels of transmission is used at a time and (2) a pseudo noise (PN) system in which a spread spectrum is utilized in order to disperse the signal in a single broad band channel in order to reduce the effects of interference. It would appear reasonably obvious to combine these two techniques in a single FH/PN system.
Because of the extreme bandwidths required for any of these systems, it is becoming more and more difficult to allot large enough contiguous sectors of the spectrum to such systems. FH and FH/PN systems may be operated, to some degree, in smaller multiple and non-contiguous sectors but this is not possible for the PN systems. This concept of spectrum assignment wil be referred to as "frequency management", herein.
Certain hypothetical parameters will be assumed throughout the balance of this disclosure for purposes of illustration only. It will be assumed that the frequency range within which the transmission of datalink communications must occur is from 12 to 16 GHz. The data rate will be assumed to be 0.5 megabits per second with a bit error rate (BER) of less than 10.sup.-5. If a single channel system were to be used, the bandwidth would be assumed to be limited to 1 GHz null to null, the code chip rate (in a PN system) 500 megachips per second; the processing gain, 500 megachips per 0.5 megabits equalling 1000, or 30 db. It will be further assumed that the total average radiated power must be at least 25 db above a noise-free environment in order to assure reliable communications. In current state-of-the art systems (above) the single channel bandwidth is indicated at 1 GHz. It is probable that as the spectrum becomes more and more crowded with competing electronic signals it will become more and more difficult to allot as much as 1 GHz to a single channel in a contiguous portion of the spectrum. If as many as four systems are employed, that is, the four systems occupy four different one GHz ranges within the 4 GHz alloted for the purpose, it may be seen immediately that the 4 GHz spectrum is completely filled. It is even less realistic to expect that the full 4 GHz spectrum will be available for such transmissions. The frequency management problem is, at least, severe.
It is also desirable that the system use as little spectrum as is necessary to accomplish the end result required, namely, good data communications. In those applications where interference-free communications are required, it is another desirable aspect of such a system to reduce detection susceptibility. The system should deny intelligence of the utilized spectrum to those who would interfere. If, at the same time, the power demands of the system can be reduced, this is also desirable.
Single channel systems also demonstrate fade margin problems. That is, since any single channel system is susceptible to varying power levels at the input of the receiver thereof, it is necessary to operate at increased power to guarantee reliable communications under worst case fade conditions. Clearly it would be helpful if system power could be reduced under these conditions.
Another technique commonly used to reduce the effect of man-made noise in a datalink system is the employment of a null steering antenna in order to minimize the noise input with respect to the required or desired signal input. Such systems are complex and expensive and clearly it would be of advantage to eliminate such devices from such a system.